The use of diaphragms in piston-driven fluid pumps is well known. Diaphragm pumps offer several advantages over conventional piston pumps, among which are resistance to leakage and a decrease in the number of parts required. In such pumps, the diaphragms are not driven directly, but are driven through a hydraulic pressure medium or "drive fluid", commonly oil, which is contained within a drive chamber and pressurized by a piston arrangement. The diaphragm forms a boundary between the drive fluid and the fluid to be pumped. In order to provide a safety mechanism against overpressurization of the drive fluid, the drive chamber may be connected to a dump valve, which permits the escape of drive fluid from the drive chamber when the drive fluid exceeds a predetermined maximum pressure.
One disadvantage of known diaphragm pumps is that the drive fluid, through actuation of the dump valve or passage through a cooling mechanism, may be exposed to the ambient atmosphere. This exposure not only increase the risk of contamination of the drive fluid, but permits air to be mixed with the drive fluid. During subsequent operation of the pump, air within the drive fluid causes carbonization or oxidation to occur, which shortens the effective life of the drive fluid, and may cause premature failure of the pump itself.
Another disadvantage of known diaphragm pumps is that, when the dump valve is actuated, pressure within the drive chamber drops to atmospheric pressure. Until drive fluid is restored to the drive chamber, continued operation of the piston results in undesirable cavitation within the pump.
Yet another disadvantage of known diaphragm pumps is the risk of contamination of pump fluid by leaking drive fluid. If the diaphragm seal is defective, or develops a leak due to wear, drive fluid may escape to the pump chamber, where it may commingle with, and contaminate, fluid to be pumped. Such contamination is particularly undesirable in applications where the purity of the fluid to be pumped is critical.
Yet another disadvantage of known diaphragm pumps is that, in most arrangements, the only way to vary the pressure or flow rate of fluid through the pump is to vary the drive speed of the piston. This often means that the motor driving the piston is operated at less-than-optimal efficiency.
Still another disadvantage of known diaphragm pumps is that they tend to provide a pulsating flow of pumped liquid. In order to counteract such pulsation, various damping arrangements have been proposed (see e.g. U.S. Pat. No. 4,459,089 to Vincent et al.). However, known damping arrangements are relatively complex, thus negating some of the inherent advantages of diaphragm pumps.